Explanation:

HDI stands for Human Development Index, which is a composite index used to measure the overall development of a country. It was developed by the United Nations Development Programme (UNDP) in 1990. HDI combines three basic dimensions of human development: longevity, knowledge, and standard of living. It is a measure of how well people are living in a country.

The three dimensions of HDI are:

1. Longevity: It refers to the life expectancy at birth of a person in a country. It is one of the basic indicators of human development. Higher life expectancy indicates better health facilities and living conditions in a country.

2. Knowledge: It refers to the education level of people in a country. It is measured by two indicators: mean years of schooling and expected years of schooling. Mean years of schooling refer to the average number of years that a person aged 25 or older has spent in school. Expected years of schooling refer to the number of years a child of school entrance age can expect to spend in school.

3. Standard of living: It refers to the economic well-being of people in a country. It is measured by Gross National Income (GNI) per capita. It takes into account the purchasing power of a country's currency as well.

Conclusion:

Therefore, the correct answer is option 'C', which is life expectancy. HDI does not consist of life expectancy as a dimension, but it is used as an indicator to measure the longevity dimension of HDI.